Check engine to find the code using the paperclip method. As to what can illuminate the CEL, there are several items. One (i recently dealt with) was caused by a faulty high temp sensor. As was stated, this is different than the temp sender the ecu reads to determine if engine is hot or cold. If that sensor is reading cold when engine is warmed up, it will run too rich but i believe this feeds the dash guage too so probably is OK. . Knock sensor could be retarding timing if bad as well. You can buy one of those adhesive timing tapes from a speed shop that you put on the harmonic damper and check with a timing light if advance is working for ~ $10 if you want to 100% rule this out.

Sorry to hear you are having so many problems. Usually these boats are pretty trouble free ( provided they recieve the proper maintenance) but sometimes a boat just turns out to be a turd no matter how well maintained. No different than buying a car and getting a lemon.

One point in you note still puzzles me- why the PO was adjusting the fuel pressure regulator? These boats are FI so could be that the injectors are leaking by which can cause a rich idle if the temp sensor turns out to be OK. You may want to look in the throttle bore while at idle and at speed to see if they are both working correctly as well. These were used on tons of early chevys so you can look for ideas on the web there to get ideas. I remember one post where the injector was cutting out at high rpm due to loose wiring connection.